This invention relates to an electronic control type fuel injection system which stops fuel injection when decelerating an engine, and particularly to the improvement of fuel injection in the resumption of the fuel injection from stopping of fuel injection to normal state thereof.
In the conventional fuel injection apparatus, a throttle sensor is provided in the throttle valve of engine to detect whether the opening degree of the throttle valve is less than a predetermined value or not. If the opening degree is less than the predetermined value (when the idle switch is in the on-state) and the engine speed is higher than a preset value N.sub.1, fuel injection to engine is stopped, and then when the throttle valve is opened to a preset opening degree or above (the idle switch is in the off-state) or when the throttle valve is opened to less than the preset opening degree (the idle switch is in the on-state) and the engine speed is reduced to the preset rotational rate N.sub.1 or below, resumption of fuel injection to supply the fuel again from the fuel stopped state is carried out.
As the method of resuming fuel supply when the engine speed is reduced to N.sub.1 or below under the on-condition of the idle switch, there are two ways as follows:
(i) The pulse width is determined by the normal calculation; and
(ii) As is known in the Japanese Laid-Open Publication No. 54-55237 published in 1979 and based on an application the priority of which is claimed in U.S. Pat. No. 4,221,193, a pulse for fuel supply whose pulse width is reduced to be narrower than that determined by the normal calculation first in the fuel injection resumption and then the value of reduction is gradually decreased in the laspe of time until the normal value is reached as it is called the reduction control at the time of fuel injection resumption.
The method (i) has the following drawbacks. Since the normal amount of fuel is injected immediately after cancellation of fuel injection stop, fuel combustion is started suddenly, therefore, the engine torque is increased rapidly, thus it makes the drive feeling poor. This phenomenon has a great effect on the feeling when the rotational rate of engine is low, and therefore the fuel supply cut rotational rate N.sub.1 must be set up to be high for preventing from that the drive feeling becomes deteriorated. This setting of the rotational rate N.sub.1 reduces the effects of such great advantages as low fuel consumption effect, low discharge of harmful exhaust gas, and low heat load of catalizer. In the method (ii) which is made in view of the problem of the drawbacks of the method (i), although the drive feeling during the normal decelerating running in the clutch-on state is improved, following drawbacks may occur, that is, when the rate of change of the engine speed is great as in the racing time (that is, when engine is driven while clutch is off), and when fuel amount reduction at resumption after cancellation of fuel injection stop is performed upon rapid decrease of engine speed, much time is required until the normal air-fuel ratio is restored to and insufficient combustion continues until the restoration of the ratio so that enough torque is not obtained, resulting in great reduction of the engine speed, in which case the reduced engine speed may not be able to be restored to the original value even if the normal air-fuel ratio is reached, and an engine stall is liable to occur. Therefore, the upon-fuel-cut speed N.sub.1 is required to be inevitably increased like in the method (i) and thus has the same defect as in the method (i).